Abstract:
We report on the design and characterization of two full height waveguide
SIS mixers for astronomical applications: a Double Side Band (DSB) fixed-tuned
mixer covering the 225-370 GHz band (
of relative
bandwidth), and a tunable Single Side Band (SSB) mixer
covering the 247-360 GHz frequency range. The DSB receiver noise temperature
we have measured is below 50 K over a bandwidth larger than 100 GHz
for the DSB mixer and has a minimum of 27 K (uncorrected) at 336 GHz;
to our knowledge this is the lowest noise ever reported at this
frequency. A receiver noise temperature below 80 K and an image band
rejection around -14 dB were measured over most
of the band of the SSB mixer.
Both mixers use similar chips that integrate
a parallel tuning inductor with a radial microstrip stub to compensate
for the junction capacitance of 75 fF (junction size
). A stability criterion for intrinsically DSB and SSB mixers
under typical operating conditions has been derived. The receiver designs
have been optimised in order to guarantee a low mixer noise
temperature while maintaining adequate gain and stable operation over
the whole frequency bands of interest.

Abstract:
We present a high-resolution (
)
image of the nucleus of
M82 showing the presence of widespread emission of the formyl radical
(HCO). The HCO map, the first obtained in an external galaxy, reveals
the existence of a structured disk of
pc full
diameter. The HCO distribution in the plane mimics the ring morphology
displayed by other molecular/ionized gas tracers in M82. More
precisely, rings traced by HCO, CO, and H II regions are nested, with
the HCO ring lying in the outer edge of the molecular
torus. Observations of HCO in Galactic clouds indicate that the
abundance of HCO is strongly enhanced in the interfaces between the
ionized and molecular gas. The surprisingly high overall abundance of
HCO measured in M82 [
]
indicates that its nuclear disk
can be viewed as a giant photon-dominated region (PDR) of
pc
size. The existence of various nested gas rings, with the highest HCO
abundance occurring at the outer ring [
],
suggests that
PDR chemistry is propagating in the disk. We discuss the inferred
large abundances of HCO in M82 in the context of a starburst
evolutionary scenario, picturing the M82 nucleus as an evolved
starburst.
Based on observations carried out with the Institut de
Radioastronomie Millimétrique (IRAM) Plateau de Bure
Interferometer. IRAM is supported by the Institut National des
Sciences de l'Univers/Centre National de la Recherche Scientifique
(France), the Max-Planck-Gesellschaft (Germany), and the Instituto
Geográfico Nacional (Spain).

Abstract:
We present images of the J=10-9 rotational lines of HC3N in the
vibrationally excited levels 1v7, 1v6, and 1v5
of the hot core (HC) in
Orion KL. The images show that the spatial distribution and the size
emission from the 1v7 and 1v5 levels are different. While the J=10-9
1v7 line has a size of
and peaks
northeast of the 3 mm continuum peak, the J=10-9 1v5 line
emission is unresolved (
)
and peaks
south of the 3 mm peak. This is a clear indication that the
HC is composed of condensations with very different temperatures (170
K for the 1v7 peak and >230 K for the 1v5 peak). The temperature
derived from the 1v7 and 1v5 lines increases with the projected
distance to the suspected main heating source I. Projection effects
along the line of sight could explain the temperature gradient as
being produced by source I. However, the large luminosity required for
source I (
)
to explain the 1v5 line suggests that
external heating by this source may not dominate the heating of the
HC. Simple model calculations of the vibrationally excited emission
indicate that the HC can be internally heated by a source with a
luminosity of
,
located 1.2
southwest of the 1v5 line peak
(1.8
south of source I). We also report the first detection of
high-velocity gas from vibrationally excited HC3N emission.
Based on excitation arguments, we conclude that the main heating
source is also driving the molecular outflow. We speculate that
all the data presented in this Letter and the IR images are
consistent with a young massive protostar embedded in an edge-on disk.

Abstract:
We used the IRAM Interferometer to detect CO(3-2), CO(7-6), and 1.3 mm
dust continuum emission from the submillimeter galaxy SMM J14011+0252
at a redshift of 2.6. Contrary to a recent claim that the CO was
extended over
(57 kpc), the new data yield a size of
for the CO and the dust. Although previous results placed
the CO peak in a region with no visible counterpart, the new maps show
the CO and dust are centered on the J1 complex seen on K-band and
optical images. We suggest the CO is gravitationally lensed not only
by the foreground cluster A1835, but also by an individual galaxy on
the line of sight. Comparison of measured and intrinsic CO brightness
temperatures indicates the CO size is magnified by a factor
of .
After correcting for lensing, we derive a true CO diameter of
(700 pc),
consistent with a compact circumnuclear disk of warm
molecular gas similar to that in Arp 220. The high magnification means
the true size, far-IR luminosity, star formation rate, CO luminosity,
and molecular gas mass are all comparable with those in present-epoch
ultraluminous IR galaxies, not with those of a huge, massive,
early-universe galactic disk.

Abstract:
Recently, we have developed and calibrated the Synthetic Field Method to
derive the total extinction through disk galaxies. The method is based on
the number counts and colors of distant background field galaxies that can
be seen through the foreground object; it is the only method capable
of determining extinction without a priori assumptions about the dust
properties or its spatial distribution, and has been successfully applied
to NGC 4536 and NGC 3664, two late-type galaxies located, respectively, at
16 and 11 Mpc.

Here, we study the applicability of the Synthetic Field Method to HST images
of galaxies in the Local Group, and show that background galaxies cannot be
easily identified through these nearby objects, even with the best resolution
available today. In the case of M 31, each pixel in the HST images contains
fifty to one hundred stars, and the background galaxies cannot be seen because
of the intrinsic granularity due to strong surface brightness fluctuations. In
the LMC, on the other hand, there is only about one star every six linear
pixels, and the lack of detectable background galaxies results from a
``secondary'' granularity, introduced by structure in the wings of the point
spread function.

The success of the Synthetic Field Method in NGC 4536 and NGC 3664 is
a natural consequence of the reduction of the intensity of surface
brightness fluctuations with distance. When the dominant confusion
factor is structure in the PSF wings, as is the case of HST images
of the LMC, and would be the case of M 31 images obtained with a 10-m
diffraction-limited optical telescope, it becomes in principle
possible to improve the detectability of background galaxies by
subtracting the stars in the foreground object. However, a much better
characterization of optical PSFs than is currently available would be
required for an adequate subtraction of the wings. Given the
importance of determining the dust content of Local Group galaxies,
efforts should be made in that direction.

Abstract:
2MASS K
band data of the inner 60 pc of the Galaxy are used
to reconstruct the line-of-sight distances of the giant molecular
clouds located in this region. Using the 2MASS H band image of the
same region two different populations of point sources are identified
according to their flux ratio in the two bands. The population of blue
point sources forms a homogeneous foreground that has to be subtracted
before analyzing the K
band image. The reconstruction is
made using two basic assumptions: (i) an axis-symmetric stellar
distribution in the region of interest and (ii) optical thick clouds
with an area filling factor of 1 that block all light of stars
located behind them. Due to the reconstruction method the relative
distance between the different cloud complexes is a robust result,
whereas the absolute distance of structures located more than 10 pc in
front of Sgr A* might be up to 30% larger than the one we
derived from the data. It is shown that all structures observed at
1.2 mm continuum and in the CS(2-1) line are present in absorption. We
place the 50 kms-1 cloud complex close to, but in front of
Sgr A*. The 20 kms-1 cloud complex is located in front of
the 50 kms-1 cloud complex and has a large LOS distance
gradient along the direction of the galactic longitude. The
Circumnuclear Disk is not seen in absorption.
This leads to an upper limit of the cloud sizes within the
Circumnuclear Disk of 0.06 pc.